The National Union of Teachers is under pressure to do an embarrassing U-turn on its threat to boycott Sats after a ballot of members drew responses from only 25% of its membership, the Guardian has learned.

Some 95% of the respondents said they wanted to see Sats scrapped, but only 75% backed a boycott, which would see the Sats tests grind to a halt and a major stand-off with the government in the weeks leading up to the general election.

The NUT insisted that it was a sound turnout. The executive of the union is meeting this afternoon to decide what action to take next.

John Dixon, assistant secretary of organisation and membership at the NUT, said: "This is a good turnout. We balloted all of our members – even those who won't be involved in the action. Our analysis shows that 35% of people in leadership positions voted, which is significant. This ballot is an overwhelming rejection of the Sats regime."

The National Association of Head Teachers, the union that represents many of the heads in primary schools, has already voted to back a boycott of the tests, but is unlikely to go ahead without the NUT.

The tests for 11-year-olds in English, maths and science were introduced in 1995 and have always been controversial. Much of the opposition to the tests stems from the use of results to create league tables.

Ed Balls, the schools secretary, has announced reforms to next year's tests which will see teacher assessments published alongside the externally marked tests. If these prove popular with parents, Sats could be scrapped by 2011. Balls has claimed that this move is backed by the unions, but Mick Brookes, head of the NAHT, said he would not support anything that included league tables.